1. Field on the Invention
The present invention relates to a semiconductor type dynamical quantity sensor device and, more particularly, to a differential capacitor type semiconductor sensor device, which may be used as an accelerometer device.
2. Related Art
In a conventional differential capacitor type semiconductor accelerometer device 11, as shown in FIGS. 22 and 23, a weight portion 15 and a comb-shaped movable electrode 16 are formed integrally on a semiconductor layer of a semiconductor substrate (Si) 19 to provide a movable unit 12. A pair of comb-shaped fixed electrodes 17 and 18 are formed also on the semiconductor substrate 19 through an insulator layer (SiO.sub.2) 21 to face the movable electrode 16. The movable electrode 16 and the fixed electrodes 17 and 18 are spaced apart and electrically insulated by a trench formed on the semiconductor substrate 19 to provide capacitors between detection surfaces thereof facing each other. The movable unit 12 is supported at both ends thereof by the semiconductor substrate 19 and movable in an axial direction of the movable unit 12 (in up-down direction in FIG. 22) in response to acceleration exerted thereon to change the capacitance between the movable electrode 16 and the fixed electrodes 17 and 18.
In this accelerometer device 11, the electrodes 16, 17 and 18 are in plate shape and have respective self-weights. As a result, the electrodes 16, 17 and 18 are likely to be broken by the respective self-weights when a large acceleration is exerted in a direction (up-down direction in FIG. 23) perpendicular to the direction of acceleration to be detected (up-down direction in FIG. 22). If the electrode width is narrowed to reduce the respective self-weights, the strength of the electrodes against the torsion or twist force will be lessened.
Further, in this accelerometer device 11, parasitic capacitors CP1, CP2 and CP3 are formed in addition to capacitors CS1 and CS3 between the movable electrode 16 and the fixed electrodes 17 and 18 as shown in FIG. 24. Specifically, the capacitors CP1, CP2 and CP3 are formed between a connecting part 170 of the fixed electrode 17 and the substrate 19, between a connecting part 180 of the fixed electrode 18 and the substrate 19 and between the movable electrode 16 and the substrate 19, respectively. The capacitors CS1 and CS3 are variable in response to the movement of the movable unit 12.
The capacitance changes of the capacitors CS1 and CS2 caused by the acceleration may be detected by a switched capacitor circuit 10 connected to pads 28, 29 and 30 of the accelerometer device 11 as shown in FIG. 25. Specifically, the switched capacitor circuit 10 comprises an amplifier AMP, a capacitor Cf and an on/off switch SW. The circuit 10 is designed to operate differentially to produce an output voltage Vo when carrier wave voltages CWV1 and CWV2 are applied as shown in FIG. 26. The output voltage Vo is expressed as follows. EQU Vo=[(CS1-CS2)+(CP1-CP2).times.CP3].times.V.div.Cf
As long as the capacitance of the parasitic capacitors CP1 and CP2 are equal to each other, the output voltage Vo varies solely in accordance with changes in capacitance of the capacitors CS1 and CS2. However, if the position of etching the substrate 19 varies as shown by the dotted line in FIG. 24, the parasitic capacitor CP1 becomes larger than the parasitic capacitor CP2. This difference in the parasitic capacitors CP1 and CP2 causes an offset voltage, which is a deviation of the output voltage Vo from zero, even when no acceleration is applied.